What's an easy way to round out this castle wall?
You don´t need to do that. Just use a cylinder for the tower. Select the upper face and extruded out to extend it a little and then extrude it up to give heigh.Make two loop cuts one outside and one inside the extruded faces to create a division for the up and down wall ( i don´t know the exact name of that part) Then jus select the faces you need to make the upper part of the tower and erase them. After that select the edges of the adjacent face and click on keyboard F to fill it with a face.